In manufacturing press parts of automobiles, two or more kinds of parts are integrally formed so as to simplify manufacturing processes and reduce the number of dies. In case of producing such united parts from raw plates, many scraps are also produced undesirably. Thus, the method of continuously welding sheet plates of the same material by the laser welding, mash seam welding, electron beam welding, TIG welding, arc welding, etc. and press-forming the resulting article integrally has been developed so as to increase the yield of the raw plate. Furthermore, taking safety at the time of collision into consideration, a tailored blank article consisting of multiple different plates is widely used. Such a tailored blank article comprises raw plates each having a different strength required for a part of the parts and different thickness and being welded continuously.
Such a tailored blank article whose plates are joined together by continuous welding has the above-described economic effect. On the other hand, there is raised a problem of defective forming at the time of press forming due to material deterioration at continuously welded portions. Breaks at the time of press forming are classified into the “ductility rate-determining mode” and the “stress rate-determining mode”. In the “ductility rate-determining mode”, when a blank plate is stretched parallel to welded bead portions, welded bead portions with material deterioration comes into a break. In the “stress rate-determining mode”, when a blank plate is stretched while pinching the welded bead portions, the raw plate having lower strength comes into a base plate break.
To cope with such situations, a steel plate fulfilling 2.6≦f(C, Si, Mn, P, B)≦12.5 is invented and described in Japanese Patent Laid-Open Publication No. Hei. 7-26346 as an ultra-low-carbon steel plate excellent in formability after high density energy beam welding. However, it becomes clear that use of such an ultra-low-carbon steel plate may not attain necessary strength required for a part to which high strength raw plates are applied in these days, and that there is no effect against a break in the “stress rate-determining mode” even though there is an effect against a break in the “ductility rate-determining mode” due to improvement in properties of welded bead portions.
In this point, concerning strain distribution when a break occurs in the stress rate-determining mode, strain ratio of two or more kinds of raw plates can be obtained by elementary analysis using strength ratio of the raw plates from conventional art (for example, Plasticity and Working, Vol. 32, No. 370 (1991) 1383 to 1390 by Kouichi Ikemoto and others). The relational expression of stress-strain of two kinds of materials can be shown by σ1=K1ε1n1, σ2=K2ε2n2 where suffix 1 is for a plate having a high strength and suffix 2 is for a plate having a low strength. At a joined portion, the relation of σ1t1=σ2t2 holds since stress between the plates is balanced. Solving these equations, when a low strength plate reaches break limit, strain (ε1max) of a high strength plate is given by the following equation (1) using TS1 and TS2.ε1max=n1{(t2/t1)(TS2/TS1)(exp(n2)/(exp(n1))}1/n1  (1) where                a: tensile stress [MPa]        K: plasticity coefficient [MPa]        ε: logarithm plastic strain        n: work hardening exponent        TS: maximum tensile strength [MPa]        
However, even though the maximum strain of a high strength plate can be calculated, there is no description about a method of solving the problem of a break in the “stress rate-determining mode.” Thus, at building sites of press forming, in case the “stress rate-determining mode” of a tailored blank article consisting of multiple different plates occurs, it is required that plate thickness ratio be decreased so as to decrease raw plate strength ratio, or strength ratio be decreased.